Cathodic Materials for Use in Electrochemical Sensors and Associated Devices and Methods of Manufacturing the Same

ABSTRACT

A cathodic material for use in an electrochemical sensor comprising: a carbonaceous material and an oxygen reduction catalyst associated with the carbonaceous material; and wherein the cathodic material does not materially exhibit catalytic activity for the oxidation of carbon monoxide. Associated electrochemical sensors may include an anode and cathode that are disposed upon the same or opposite sides of an ion exchange membrane and/or exposed to the same or different gaseous environments.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.12/804,977, filed Aug. 3, 2010, entitled “Cathodic Materials For Use InElectrochemical Sensors And Associated Devices And Methods OfManufacturing The Same,” which claims the benefit of U.S. ProvisionalApplication Ser. No. 61/231,229, filed Aug. 4, 2009, entitled “CathodicMaterials For Use In Electrochemical Sensors And Associated Devices,”which are hereby incorporated herein by reference in their entirety,including all references cited therein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to cathodic materials for usein fuel cells and electrochemical sensors and, more particularly, tocarbonaceous materials associated with oxygen reduction catalysts, forexample, substituted and unsubstituted transition metal porphyrins,substituted and unsubstituted transition metal tetrabenzoporphyrins,substituted and unsubstituted transition metal tetraphenylporphyrins,substituted and unsubstituted transition metal tetraazaporphyrins,substituted and unsubstituted transition metal tetraazamacrocycles,substituted and unsubstituted transition metal phthalocyanines,substituted and unsubstituted transition metal naphthalocyanines,substituted and unsubstituted transition metal bis(phthalocyanines),substituted and unsubstituted transition metal bis(naphthalocyanines),and combinations thereof. The present invention further relates to fuelcells and electrochemical sensors having novel structuralconfigurations, wherein the anode and the cathode of an associated microelectrode assembly (MEA) are disposed upon the same side or differentsides of an ion exchange membrane, and/or exposed to the same gaseousenvironment or different gaseous environments—among otherconfigurations. The present invention also relates to gas, smoke and/orfire detectors comprising novel electrochemical sensors, micro electrodeassemblies, and/or sub-components for the same, as well as associatedmethods of manufacturing.

2. Background Art

Fuel cells and electrochemical sensors for use in, for example, gas,smoke, and/or fire detectors have been known in the art for severalyears. See, for example, U.S. Pat. No. 4,329,214 entitled “Gas DetectionUnit,” U.S. Pat. No. 5,302,274 entitled “Electrochemical Gas SensorCells Using Three Dimensional Sensing Electrodes,” U.S. Pat. No.5,331,310 entitled “Amperometric Carbon Monoxide Sensor Module forResidential Alarms,” U.S. Pat. No. 5,573,648 entitled “Gas Sensor Basedon Protonic Conductive Membranes,” U.S. Pat. No. 5,618,493 entitled“Photon Absorbing Bioderived Organometallic Carbon Monoxide Sensors,”U.S. Pat. No. 5,650,054 entitled “Low Cost Room TemperatureElectrochemical Carbon Monoxide and Toxic Gas Sensor with HumidityCompensation Based on Protonic Conductive Membranes,” U.S. Pat. No.5,944,969 entitled “Electrochemical Sensor With A Non-AqueousElectrolyte System,” U.S. Pat. No. 5,958,200 entitled “ElectrochemicalGas Sensor,” U.S. Pat. No. 6,172,759 entitled “Target Gas DetectionSystem with Rapidly Regenerating Optically Responding Sensors,” U.S.Pat. No. 6,200,443 entitled “Gas Sensor with a Diagnostic Device,” U.S.Pat. No. 6,936,147 entitled “Hybrid Film Type Sensor,” U.S. Pat. No.6,948,352 entitled “Self-Calibrating Carbon Monoxide Detector andMethod,” U.S. Pat. No. 7,077,938 entitled “Electrochemical Gas Sensor,”U.S. Pat. No. 7,022,213 entitled “Gas Sensor and Its Method ofManufacture,” U.S. Pat. No. 7,236,095 entitled “Solid State Sensor forCarbon Monoxide,” U.S. Pat. No. 7,279,081 entitled “ElectrochemicalSensor,” U.S. Patent Publication No. 2005/0145494 entitled “LiquidElectrochemical Gas Sensor,” U.S. Patent Publication No. 2006/0091007entitled “Gas Detecting Device with Self-Diagnosis for ElectrochemicalGas Sensor,” U.S. Patent Publication No. 2006/0120924 entitled “ProtonConductor Gas Sensor,” and U.S. Patent Publication No. 2006/0196770entitled “Liquid Electrochemical Gas Sensor,” all of which are herebyincorporated herein by reference in their entirety—including allreferences cited therein.

While the utilization of fuel cells and electrochemical sensors for usein gas, smoke, and/or fire detectors has become increasingly popular,sensor performance, cost, longevity, and/or configuration remainslargely problematic.

Indeed, modern fuel cells and electrochemical sensors commonly usecarbon supported noble metal catalysts, such as platinum at both theanode and the cathode. At the anode, platinum typically catalyses theoxidation of the fuel, such as hydrogen, methanol, carbon monoxide,etcetera. At the cathode, platinum typically catalyses the reduction ofoxygen. For example, the chemical reactions that typically occur in anelectrochemical carbon monoxide sensor are provided below:

Anode: CO+H₂O→CO₂+2H⁺+2e ⁻

Cathode: ½O₂+2H⁺+2e ⁻→H₂O

Net: CO+½O ₂→CO₂.

As is shown in FIG. 10, due to the identical nature of the catalystelectrodes, without biasing the electrodes, electric current generation(e⁻) can typically only occur if both the anode and the cathode areseparated and sealed from each other to prevent gas crossover. The twoelectrodes are typically separated by a membrane that allows fordiffusion of ions, such as protons (H⁺), and water (H₂O). Gas crossoverthrough the membrane is possible, but the diffusion rate of the samplegas is controlled so that the fuel gas (H₂, CO, etcetera) is effectivelyscrubbed from the sample gas by the anode and only oxygen is allowed tocrossover to the cathode.

When both the anode and the cathode comprise identical materials, inthis case carbon supported platinum, there can be several problems. Thefirst occurs when the gas diffusion is not controlled and the gasescrossover from one electrode to the other. This can lead to degradationof the electrodes through peroxide formation or oxidation. In anelectrochemical sensor, gas crossover results in reduced signal strengthand polarization of the sensor which can potentially lead to sensormalfunction. Platinum is sensitive to poisoning from externalcontaminants, such as sulfur compounds, which reduces the electricalcurrent being generated. Electrode sensitivity can also drop over timedue to reduced surface area of the platinum particles caused byrearrangement and sintering. Additionally, amorphous carbon which is acommon material used as a carbon support, (e.g., XC72 (CabotCorporation) or Black Pearls (Cabot Corporation)) is susceptible tooxidation. The oxidation reaction can even be catalyzed by the verymaterials that the carbon is meant to support, such as Pt, Ru, Pd,etcetera. This oxidation can result in the presence of a backgroundcurrent in a sensor application, reduced electrical conductivity withinthe electrodes, and migration and aggregation of metal nanoparticlesresulting in reduced power output or sensitivity. This is especiallyproblematic for a sensor application in which long term drift andreduced sensitivity can be catastrophic. Finally, platinum is a noblemetal that is rare and expensive.

It is therefore an object of the present invention, among other objects,to provide novel anodic and/or cathodic materials which replaceconventional platinum based electrodes in fuel cells and electrochemicalsensors. It is also an object of the present invention to provide noveldevice configurations which are enabled by the use of these novelmaterials for the anode and/or cathode.

These and other objects of the present invention will become apparent inlight of the present specification, claims, and drawings.

SUMMARY OF THE INVENTION

The present invention is directed to a cathodic material for use in anelectrochemical sensor comprising: a carbonaceous material and an oxygenreduction catalyst associated with the carbonaceous material, whereinthe cathodic material does not materially exhibit catalytic activity forthe oxidation of carbon monoxide. The carbonaceous material preferablyacts as a support and/or an electron conductor. The cathodic material isalso optionally associated with an ion-exchange material, such as protonconducting Nafion (Dupont), polystyrene sulfonic acid (Sigma-Aldrich),H₃PO₄ doped polybenzimidazole derivatives (L. Xiao, et al., Fuel Cells 5(2005) 287), protic ionic liquid doped polybenzimidazole derivatives,and protic ionic liquid doped sulfonated polyimide derivatives (S.-Y.Lee, et al., J. Power Sources (2009)), etcetera.

In a preferred embodiment of the present invention, the oxygen reductioncatalyst comprises a material resulting from pyrolysis of at least oneof the group comprising substituted transition metal (i.e., d-block)porphyrins, unsubstituted transition metal porphyrins, substitutedtransition metal tetrabenzoporphyrins, unsubstituted transition metaltetrabenzoporphyrins, substituted transition metaltetraphenylporphyrins, unsubstituted transition metaltetraphenylporphyrins, substituted transition metal tetraazaporphyrins,unsubstituted transition metal tetraazaporphyrins, substitutedtransition metal tetraazamacrocycles, unsubstituted transition metaltetraazamacrocycles, substituted transition metal phthalocyanines,unsubstituted transition metal phthalocyanines, substituted transitionmetal naphthalocyanines, unsubstituted transition metalnaphthalocyanines, substituted transition metal bis(phthalocyanines),unsubstituted transition metal bis(phthalocyanines), substitutedtransition metal bis(naphthalocyanines), unsubstituted transition metalbis(naphthalocyanines), and/or combinations thereof.

In another preferred embodiment of the present invention, the oxygenreduction catalyst comprises a material resulting from pyrolysis of atleast one of the group comprising substituted cobalt porphyrins,unsubstituted cobalt porphyrins, substituted cobalttetrabenzoporphyrins, unsubstituted cobalt tetrabenzoporphyrins,substituted cobalt tetraphenylporphyrins, unsubstituted cobalttetraphenylporphyrins, substituted cobalt tetraazaporphyrins,unsubstituted cobalt tetraazaporphyrins, substituted cobalttetraazamacrocycles, unsubstituted cobalt tetraazamacrocycles,substituted cobalt metal phthalocyanines, unsubstituted cobaltphthalocyanines, substituted cobalt naphthalocyanines, unsubstitutedcobalt naphthalocyanines, substituted cobalt bis(phthalocyanines),unsubstituted cobalt bis(phthalocyanines), substituted cobaltbis(naphthalocyanines), unsubstituted cobalt bis(naphthalocyanines),and/or combinations thereof.

In yet another preferred embodiment of the present invention, the oxygenreduction catalyst comprises a material resulting from pyrolysis of atleast one of a compound, a structural isomer of a compound, and mixturesof isomers of compounds, represented by the following structure:

wherein M comprises a transition metal ligated by a tetraazamacrocycle,including, but not limited to, substituted porphyrins, unsubstitutedporphyrins, substituted phthalocyanines, unsubstituted phthalocyanines,substituted naphthalocyanines, unsubstituted naphthalocyanines, andcombinations thereof—just to name a few.

In one embodiment of the present invention, the oxygen reductioncatalyst comprises a material resulting from pyrolysis of at least oneof a compound, a structural isomer of a compound, and mixtures ofisomers of compounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₆ are the same or different and comprise H, NO₂, NH₂, NHR₁₇,N(R₁₈)₂, CO₂H, CO₂R₁₀, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₂₀, SH, SR₂₁, and combinationsthereof; and wherein R₁₇₋₂₁ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s). In this embodiment, the oxygen reduction catalyst preferablycomprises a material resulting from pyrolysis of one or more of acompound, a structural isomer of a compound, and mixtures of isomers ofcompounds, represented by the following formulae:

In a preferred embodiment of the present invention, the oxygen reductioncatalyst comprises a material resulting from pyrolysis of at least oneof a compound, a structural isomer of a compound, and mixtures ofisomers of compounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₂ are the same or different and comprise H, NO₂, NH₂, NHR₁₃,N(R₁₄)₂, CO₂H, CO₂R₁₆, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₁₆, SH, SR₁₇, and combinationsthereof; and wherein R₁₃₋₁₇ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

In another preferred embodiment of the present invention, the oxygenreduction catalyst comprises a material resulting from pyrolysis of atleast one of a compound, a structural isomer of a compound, and mixturesof isomers of compounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₀ are the same or different and comprise H, NO₂, NH₂, NHR₂₁,N(R₂₂)₂, CO₂H, CO₂R₂₃, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₂₄, SH, SR₂₅, and combinationsthereof; and wherein R₂₁₋₂₅ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

In yet another preferred embodiment of the present invention, the oxygenreduction catalyst comprises a material resulting from pyrolysis of atleast one of a compound, a structural isomer of a compound, and mixturesof isomers of compounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₈ are the same or different and comprise H, NO₂, NH₂, NHR₂₉,N(R₃₀)₂, CO₂H, CO₂R₃₁, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₃₂, SH, SR₃₃, and combinationsthereof; and wherein R₂₉₋₃₃ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

In one aspect of an embodiment of the present invention, the oxygenreduction catalyst comprises a material resulting from pyrolysis of atleast one of a compound, a structural isomer of a compound, and mixturesof isomers of compounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; wherein R₁-R₈are the same or different and comprise H, NO₂, NH₂, NHR₉, N(R₁₀)₂, CO₂H,CO₂R₁₁, an alkyl group containing approximately 1 to approximately 10carbon atom(s), OR₁₂, SH, SR₁₃, and combinations thereof; and whereinR₉₋₁₃ are the same or different and comprise an alkyl group containingapproximately 1 to approximately 10 carbon atom(s).

Preferably, the carbonaceous materials associated with the presentinvention comprise, for example, graphene, graphite, amorphous carbon,carbon nanotubes, carbon fibers, and combinations thereof. The carbonnanotubes include, for example, single walled nanotubes (SWNT), doublewalled nanotubes (DWNT), and multi-walled nanotubes (MWNT), all of whichmay optionally be doped with atoms such as, but not limited to,nitrogen, boron, and/or phosphorous.

The present invention is also directed to an electrochemical sensor foruse in a gas detector comprising: a housing, wherein the housing iscapable of containing a micro electrode assembly therein (e.g., one ormore side walls, and a bottom wall cooperatively define a containmentregion for containing the micro electrode assembly), and further whereinthe housing comprises a gaseous diffusion aperture; and a microelectrode assembly which comprises an anode, a cathode comprising one ormore cathodic materials as are disclosed herein, and an ion exchangemembrane, wherein the ion exchange membrane permits ion transport (e.g.,hydrogen ions (protons), hydroxide ions, carbonate ions, etcetera)between the anode and the cathode, and further wherein the ion exchangemembrane prevents electron conduction between the anode and the cathode.The electrochemical sensor may also include a desiccant for retaining aliquid, such as water and/or a reservoir. The electrochemical sensor mayfurther comprise a configuration to prevent water from freezing, such asantifreeze or salts. The electrochemical sensor may additionallycomprise a configuration that is at least substantially waterless (SeeFIG. 1B).

In a preferred embodiment of the present invention, the anode and thecathode of the micro electrode assembly are disposed upon opposite sidesof the ion exchange membrane and/or exposed to a different gaseousenvironment (e.g., a first configuration). Alternatively, the anode andthe cathode of the micro electrode assembly are disposed upon the sameside of the ion exchange membrane and/or exposed to the same gaseousenvironment (e.g., a second configuration).

The present invention is further directed to an electrochemical sensorfor use in a gas detector comprising: a housing, wherein the housing iscapable of containing a micro electrode assembly therein (e.g., one ormore side walls, and a bottom wall cooperatively define a containmentregion for containing the micro electrode assembly), and further whereinthe housing comprises a gaseous diffusion aperture; and a microelectrode assembly which comprises an anode, a cathode, and an ionexchange membrane, wherein the anode and the cathode are disposed uponthe same side of the ion exchange membrane and/or exposed to the samegaseous environment.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention are illustrated by theaccompanying figures. It will be understood that the figures are notnecessarily to scale and that details not necessary for an understandingof the invention or that render other details difficult to perceive maybe omitted. It will be further understood that the invention is notnecessarily limited to the particular embodiments illustrated herein.

The invention will now be described with reference to the drawingswherein:

FIG. 1A of the drawings is a cross-sectional schematic representation ofan electrochemical sensor fabricated in accordance with the presentinvention, showing among other things, a solvent reservoir;

FIG. 1B of the drawings is a cross-sectional schematic representation ofa solventless (e.g., waterless) electrochemical sensor fabricated inaccordance with the present invention;

FIG. 1C of the drawings is a cross-sectional schematic representation ofan electrochemical sensor fabricated in accordance with the presentinvention, showing among other things, a top/cover filter;

FIGS. 2A-2B of the drawings are perspective and exploded perspectiverepresentations, respectively, of micro electrode assemblies fabricatedin accordance with the present invention;

FIGS. 3A-3D of the drawings are representations of micro electrodeassemblies fabricated in accordance with the present invention;

FIGS. 4A-4D of the drawings are representations of micro electrodeassemblies associated with reference electrodes fabricated in accordancewith the present invention;

FIG. 5 of the drawings is a representation of a micro electrode assemblyhaving a key member fabricated in accordance with the present invention;

FIG. 6 of the drawings is a cyclic voltammagram verifying thatembodiments of the present invention are electrochemically activetowards the reduction of oxygen;

FIG. 7 of the drawings is a cyclic voltammagram verifying that oxygenreduction catalysts of the present invention are materiallyelectrochemically inactive with regard to the oxidation of carbonmonoxide and the reduction of carbon dioxide;

FIG. 8A-8B of the drawings are two-dimensional plots showing sensorperformance output as a function of exposure time to pulsed CO;

FIG. 9 of the drawings is a two-dimensional plot showing a linearresponse of a sensor fabricated in accordance with the presentinvention; and

FIG. 10 of the drawings is a representation of a prior art microelectrode assembly.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and to FIG. 1A in particular, across-sectional schematic representation of electrochemical (EC) sensor10 is shown, which generally comprises housing 12 and micro electrodeassembly (MEA) 14.

It will be understood that electrochemical sensor 10 may comprise, forillustrative purposes only, an electrochemical gas sensor for a gas,smoke, and/or fire detector, and the like. It will be further understoodthat FIG. 1A is merely a schematic representation of electrochemicalsensor 10. As such, some of the components may have been distorted fromtheir actual scale for pictorial clarity. Indeed, numerous otherelectrochemical cell designs and configurations are contemplated foruse, including those disclosed in U.S. Pat. No. 4,329,214 entitled “GasDetection Unit,” U.S. Pat. No. 5,302,274 entitled “Electrochemical GasSensor Cells Using Three Dimensional Sensing Electrodes,” U.S. Pat. No.5,331,310 entitled “Amperometric Carbon Monoxide Sensor Module forResidential Alarms,” U.S. Pat. No. 5,573,648 entitled “Gas Sensor Basedon Protonic Conductive Membranes,” U.S. Pat. No. 5,618,493 entitled“Photon Absorbing Bioderived Organometallic Carbon Monoxide Sensors,”U.S. Pat. No. 5,650,054 entitled “Low Cost Room TemperatureElectrochemical Carbon Monoxide and Toxic Gas Sensor with HumidityCompensation Based on Protonic Conductive Membranes,” U.S. Pat. No.5,944,969 entitled “Electrochemical Sensor With A Non-AqueousElectrolyte System,” U.S. Pat. No. 5,958,200 entitled “ElectrochemicalGas Sensor,” U.S. Pat. No. 6,172,759 entitled “Target Gas DetectionSystem with Rapidly Regenerating Optically Responding Sensors,” U.S.Pat. No. 6,200,443 entitled “Gas Sensor with a Diagnostic Device,” U.S.Pat. No. 6,936,147 entitled “Hybrid Film Type Sensor,” U.S. Pat. No.6,948,352 entitled “Self-Calibrating Carbon Monoxide Detector andMethod,” U.S. Pat. No. 7,077,938 entitled “Electrochemical Gas Sensor,”U.S. Pat. No. 7,022,213 entitled “Gas Sensor and Its Method ofManufacture,” U.S. Pat. No. 7,236,095 entitled “Solid State Sensor forCarbon Monoxide,” U.S. Pat. No. 7,279,081 entitled “ElectrochemicalSensor,” U.S. Patent Publication No. 2005/0145494 entitled “LiquidElectrochemical Gas Sensor,” U.S. Patent Publication No. 2006/0091007entitled “Gas Detecting Device with Self-Diagnosis for ElectrochemicalGas Sensor,” U.S. Patent Publication No. 2006/0120924 entitled “ProtonConductor Gas Sensor,” and U.S. Patent Publication No. 2006/0196770entitled “Liquid Electrochemical Gas Sensor,” all of which are herebyincorporated herein by reference in their entirety—including allreferences cited therein.

In accordance with the present invention housing 12 is capable ofcontaining a micro electrode assembly therein, and preferably includesfirst side wall 16, second side wall 18, and bottom wall 20, whichdefine containment region 23 for containing micro electrode assembly 14therein, among other sub-components. Housing 12 also preferably includestop wall 22 having gaseous diffusion aperture 24 which allows gassesexternal to electrochemical sensor 10 to electrochemically interact withmicro electrode assembly 14. It will be understood that while housing 12has been disclosed, for illustrative purposes only, as comprising topwall 22, gaseous diffusion aperture 24 may emanate from first side wall16 to second sidewall 18, thereby minimizing or eliminating thestructural necessity of top wall 22. It will be further understood thattop wall 22 may be fabricated from a material different than first sidewall 16 and/or second side wall 18, thereby forming a separate anddistinct housing cap. As is known in the art, housing 12 may alsoinclude filter 32, such as particulate filter and/or activated carbonfilter, which enhances the longevity and electrochemical performance ofmicro electrode assembly 14. In addition housing 12 may optionallyinclude reservoir region 34 which contains/retains solvents/fluids, suchas water and/or electrochemical cell redox agents. In a preferredembodiment of the present invention, reservoir region 34 may include adesiccant gel for retaining water or other agents that influence thechemical and/or physical properties of water. Housing 12 also preferablyincludes electrode leads 36, which enable electrical communication witha sensing circuit.

As is best shown in FIG. 1B, electrochemical sensor 10 may besolventless (e.g., waterless) and void of reservoir region 34. In thisembodiment electrochemical sensor 10 may optionally comprise an ionexchange member which preferably comprises H₃PO₄ doped polybenzimidazolederivatives, protic ionic liquid doped polybenzimidazole derivatives,and/or protic ionic liquid doped sulfonated polyimide derivatives, whichare ionically conductive regardless of the level of ambienthumidity/moisture

As is best shown in FIG. 1C, electrochemical sensor 10 may furthercomprise a top or cover filter 50 which covers gaseous diffusionaperture 24 (i.e., the sample gas inlet) and removes unwantedcontaminant gases from the sample gas that the sensor may becross-sensitive to. Additionally cover filter 50 enables the sensor tobe washed. Cover filter 50 may comprise an ePTFE (expanded Teflon suchas Gore-Tex), a charcoal impregnated felt (Calgon Carbon, Pittsburgh,Pa.), and/or a housing filled with activated charcoal powder or pellets(Calgon Carbon, Pittsburgh, Pa.; Norit Americas Inc., Marshall, Tex.).

For purposes of the present disclosure, housing 12 may be fabricatedfrom one or more of any one of a number of materials including, forexample, metals, metallic alloys, pseudo metals, natural and/orsynthetic plastics, and composites—just to name a few.

Referring now to FIGS. 1A-1C and 2A-2B, collectively, micro electrodeassembly 14 preferably includes anode 26, cathode 28, ion exchangemembrane 30, and outer substrate(s) 38 (e.g., a porous Teflon diffusionlayer). Individual current collectors 25 are preferably associated withanode 26 and cathode 28, and comprise, for example, carbonaceousconductors such as carbon based conductive paints (XCMC-40, SpraylatCorporation) or conductive carbon loaded silicone rubber (SE65Conductive Silicone, Stockwell Elastomerics, Inc.) and/or metallicconductors. It will be understood that ion exchange membrane or material30 generally permits hydrogen or other ion transport between anode 26and cathode 28, but generally prevents electron conduction between thesame. Non-limiting examples of suitable ion exchange membrane materialsinclude, for example, expanded PTFE (Gore, Porex), glass wool, and/orcellulose which may be infused with various ion conducting materialssuch as mineral acids including H₂SO₄, aqueous alkaline salts such asKOH, H₃PO₄ doped polybenzimidazole derivatives (L. Xiao, et al., FuelCells 5 (2005) 287), protic ionic liquid doped polybenzimidazolederivatives, and protic ionic liquid doped sulfonated polyimidederivatives (S.-Y. Lee, et al., J. Power Sources (2009))—all of whichare hereby incorporated herein by reference in their entirety—includingall references cited therein. Other suitable ion exchange membranematerials include solid polymeric electrolytes including, but notlimited to, polystyrene sulfonic acid (Sigma-Aldrich) Nafion (Dupont),and/or Flemion (Asahi).

Anode 26 preferably serves as a working or sensing gaseous oxidationelectrode, such as a carbon monoxide oxidation electrode which wasdiscussed supra (See for example, ¶ 0004, and FIG. 1A). Anode 26 may befabricated from any one of a number of materials including transitionmetals, alloys, and mixtures of the same. For example, platinum andplatinum-ruthenium which can be associated with a carbonaceous species,such as graphene, graphite, amorphous carbon (XC72, BlackPearls), carbonnanotubes (e.g., SWNT, DWNT, MWNT), carbon fibers, and combinationsthereof, all of which may be doped with atoms such as, but not limitedto, nitrogen, boron, and/or phosphorous—utilizing conventionaltechniques. Anode 26 may also be associated with an ion conductor, suchas Nafion in an analogous manner disclosed infra with regard to theworking examples of cathode 28. Additional suitable anodic materialsinclude those disclosed in FUEL CELL FUNDAMENTALS, 2^(nd) Ed., O'hayreet al., Wiley (2009), which is hereby incorporated herein by referencein its entirety.

Preferably cathode 28 comprises a cathodic material which includes acarbonaceous material, such as graphene, graphite, amorphous carbon,carbon nanotubes (e.g., SWNT, DWNT, MWNT), carbon fibers, andcombinations thereof, all of which may be doped with atoms such as, butnot limited to, nitrogen, boron, and/or phosphorous—utilizingconventional techniques, and an oxygen reduction catalyst associated(i.e., chemically and/or physically) with the carbonaceous material. Itwill be understood that the oxygen reduction catalysts of the presentinvention are preferably heat treated, pyrolyzed, and/or otherwiseactivated in a manner which enable the catalysts to remain functionallyoperable and stable. Notably, the cathodic materials of the presentinvention preferably do not materially exhibit catalytic activity forthe oxidation of carbon monoxide and/or the reduction of carbon dioxide.Cathode 28 may also be associated with an ion conductor, such as Nafion,in an analogous manner disclosed infra with regard to the workingexamples of the cathode. Such electrochemically selective catalystsenables traditional MEA configurations (e.g., wherein the anode and thecathode of the micro electrode assembly are disposed upon opposite sidesof the ion exchange membrane and/or exposed to different gaseousenvironments (See diagram between ¶¶ 0005-0006), as well as novelelectrode configurations wherein the anode and the cathode of the microelectrode assembly are disposed upon the same side of the ion exchangemembrane and/or exposed to the same gaseous environment (See FIGS. 1-5).

In accordance with one embodiment of the present invention, cathode 28preferably serves as a counter electrode in the MEA and participates inthe catalyzed reduction of oxygen to water which was discussed supra(See ¶ 0004).

Non-limiting examples of suitable oxygen reduction catalysts include amaterial resulting from pyrolysis of substituted, transition metal(i.e., d-block) porphyrins, unsubstituted transition metal porphyrins,substituted transition metal tetrabenzoporphyrins, unsubstitutedtransition metal tetrabenzoporphyrins, substituted transition metaltetraphenylporphyrins, unsubstituted transition metaltetraphenylporphyrins, substituted transition metal tetraazaporphyrins,unsubstituted transition metal tetraazaporphyrins, substitutedtransition metal tetraazamacrocycles, unsubstituted transition metaltetraazamacrocycles, substituted transition metal phthalocyanines,unsubstituted transition metal phthalocyanines, substituted transitionmetal naphthalocyanines, unsubstituted transition metalnaphthalocyanines, substituted transition metal bis(phthalocyanines),unsubstituted transition metal bis(phthalocyanines), substitutedtransition metal bis(naphthalocyanines), unsubstituted transition metalbis(naphthalocyanines), and/or combinations thereof. Examples ofpreferred transition metals include, but are not limited to, Co, Cu, Fe,Ir, Ni, Pd, Rh, Ru, or Zn, with Co being the most preferred transitionmetal.

Suitable oxygen reduction catalysts may also be expressed as comprisinga material resulting from pyrolysis of a compound, a structural isomerof a compound, and/or mixtures of isomers of compounds, represented bythe following structure:

wherein M comprises a transition metal which is ligated by atetraazamacrocycle, such as, for example, substituted porphyrins,unsubstituted porphyrins, substituted phthalocyanines, unsubstitutedphthalocyanines, substituted naphthalocyanines, unsubstitutednaphthalocyanines, and combinations thereof.

By way of non-limiting examples, oxygen reduction catalysts may comprisea material resulting from pyrolysis of a compound, a structural isomerof a compound, and/or mixtures of isomers of compounds, represented bythe following structure:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₆ are the same or different and comprise H, NO₂, NH₂, NHR₁₇,N(R₁₈)₂, CO₂H, CO₂R₁₀, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₂₀, SH, SR₂₁, and combinationsthereof; and wherein R₁₇₋₂₁ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

Three specific phthalocyanines which serve as oxygen reduction catalystsinclude:

By way of additional non-limiting examples, oxygen reduction catalystsmay comprise a material resulting from pyrolysis of a compound, astructural isomer of a compound, and/or mixtures of isomers ofcompounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₂ are the same or different and comprise H, NO₂, NH₂, NHR₁₃,N(R₁₄)₂, CO₂H, CO₂R₁₅, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₁₆, SH, SR₁₇, and combinationsthereof; and wherein R₁₃₋₁₇ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

In one embodiment of the present invention, the oxygen reductioncatalyst comprise a material resulting from pyrolysis of a compound, astructural isomer of a compound, and/or mixtures of isomers ofcompounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₀ are the same or different and comprise H, NO₂, NH₂, NHR₂₁,N(R₂₂)₂, CO₂H, CO₂R₂₃, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₂₄, SH, SR₂₅, and combinationsthereof; and wherein R₂₁₋₂₅ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

In accordance with another embodiment of the present invention, theoxygen reduction catalysts comprises a material resulting from pyrolysisof a compound, a structural isomer of a compound, and/or mixtures ofisomers of compounds, represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₈ are the same or different and comprise H, NO₂, NH₂, NHR₂₉,N(R₃₀)₂, CO₂H, CO₂R₃₁, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), OR₃₂, SH, SR₃₃, and combinationsthereof; and wherein R₂₉₋₃₃ are the same or different and comprise analkyl group containing approximately 1 to approximately 10 carbonatom(s).

In accordance with yet another embodiment of the present invention, theoxygen reduction catalysts comprise a material resulting from pyrolysisof a compound, a structural isomer of a compound, and/or mixtures ofisomers of compounds represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; wherein R₁-R₈are the same or different and comprise H, NO₂, NH₂, NHR₉, N(R₁₀)₂, CO₂H,CO₂R₁₁, an alkyl group containing approximately 1 to approximately 10carbon atom(s), OR₁₂, SH, SR₁₃, and combinations thereof; and whereinR₉₋₁₃ are the same or different and comprise an alkyl group containingapproximately 1 to approximately 10 carbon atom(s).

It will be understood that several of the above-identified compounds areprovided herein as working examples and that additional disclosure forthe commercial availability and/or preparation of transition metalporphyrins, tetrabenzoporphyrins, tetraphenylporphyrins,tetraazaporphyrins, tetraazamacrocycles, phthalocyanines,naphthalocyanines, bis(phthalocyanines), and bis(naphthalocyanines), areavailable from common chemical vendors, such as Sigma-Aldrich ChemicalCo., of Milwaukee, Wis. and Strem Chemical of Newburyport, Mass.

Without being bound to any one particular theory, it is believed thatthe metal atom in M-N₄ catalysts disclosed herein contribute to theproper structural formation of the most active carbon-nitrogen catalyticsites during pyrolysis. As such, removal of the metal after pyrolysisusing an acid extraction does not appear to adversely affect electrodeperformance. Suitable acid extraction techniques are provided in, forexample, (T. Ikeda, et al., J. Phys. Chem. C 112 (2008) 14706, (M. Saitoet al., 215th ECS Meeting, Abstract #265), and (M. Saito et al., 217thECS Meeting, Abstract #502)—all of which are hereby incorporated hereinby reference in their entirety—including all references cited therein.The Examples infra provide further details regarding M-N₄ catalysts.

Referring once again to FIGS. 3A-3D, electrochemical sensor 10preferably includes micro electrode assembly 14, wherein anode 26 andcathode 28 are preferably: (1) disposed upon the same plane, (2)disposed upon the same side of the ion exchange membrane, and/or (3)exposed to a same gaseous environment. It will be understood that,regardless of its ordinary meaning, the term the “same gaseousenvironment” will be defined herein as the same ambient air, and/or thesame air external to the micro electrode assembly. It will be understoodthat the micro electrode assemblies of the present invention facilitatenumerous design configurations not available heretofore. In particular,conventional cathodic materials exhibit catalytic activity for theoxidation of carbon monoxide as well as the reduction of oxygen towater. As such, without restricting the cathode to ambient sample gas,the sensor is susceptible to undesirable failure under a plurality ofconditions.

In accordance with the present invention, micro electrode assembly 14may be fabricated using any one of a number of conventional techniques,including pad or decal printing, as is disclosed in U.S. Pat. No.5,211,984, which is hereby incorporated herein by reference in itsentirety—including all references cited therein, brushing, screenprinting, spraying, ink jet printing, and/or dip coating—just to name afew.

It will be understood that a typical fuel cell or electrochemical sensorhas an ion exchange or conducting membrane that separates the twoelectrodes. Typically this membrane is approximately 10-200 μM thick.However, in certain embodiments of the present invention, ions diffuselaterally across the membrane. A long ion diffusion length increasesionic resistance and therefore reduces signal strength. To reduce anyundesirable ionic resistance, the ion diffusion length is preferablyminimized between the two electrodes, via a narrow gap, and within eachelectrode, via narrow electrodes. The cross sectional area between thetwo electrodes is also preferably maximized to enhance current flow andsignal strength. Preferentially, the MEA configuration will comprise aninterdigitated electrode design similar to that shown in FIGS. 3A and4A.

It will be further understood that a typical fuel cell orelectrochemical sensor has an electron path length that is essentiallythe thickness of each electrode which is approximately 1-100 μM, beforereaching an external current collector. However, in certain embodimentsof the present invention, electrons diffuse laterally across theelectrodes before reaching the external current collectors. As such, thecarbon based materials have a relatively high electron resistance so thepath length is preferably minimized to enhance signal strength.Preferentially, the MEA configuration comprises an interdigitatedelectrode design similar to that shown in FIG. 3A or 4A—whereinelectrode finger lengths are optimized to reduce the electron resistancebut maintain a high amount of material to maximize the rate of chemicalreactions occurring.

In accordance with the present invention, catalysts found in the anodeand cathode have different activities and levels of current outputtowards their respective gases. To reduce wasted material, among otherthings, the thickness, area, and dimensions of each electrode can bevaried. Suitable examples are provided in FIGS. 3A-3D. Preferably theratio of the two dimensional areas of the cathode to anode is greaterthan approximately 1:1, and more preferably approximately 2.4:1. Inaddition, the ratio of the thickness of the cathode to anode preferablygreater than approximately 1:1, and more preferably approximately 2:1.It will be understood that during normal operation (carbon monoxide ispresent and the sensor is sensing), H₂O is generated at the cathode. Athigh CO concentrations, the produced water can plug the porous networkof the carbon electrode thus reducing the activity of the cathode andreducing the signal strength. A cathode of large surface area can beused to wick this water away from the most active regions of theelectrode and allow a stable signal to be achieved.

As is best shown collectively in FIGS. 3-5 and Example 15, when a planardesign for micro electrode assembly 14 of sensor 10 is utilized, theaddition of MWNT to cathodic inks has provided surprisingly beneficialresults with respect to enhancing conductivity without adverselyaffecting catalytic activity.

Referring now to FIGS. 4A-4D, electrochemical sensor 10 may also includeone or more reference electrodes 40 associated with micro electrodeassembly 14. The reference electrodes facilitate, among other things,long term device performance and sensitivity optimization.

As is best shown in FIG. 5, micro electrode assembly 14 preferablyincludes key member(s) 42, which regulate orientation and positionalengagement relative to housing 12 of electrochemical sensor 10. Forexample, key member 42 may comprise one or more tabs and/or slots, whichmatingly correspond to housing 12 for proper positioning during, forexample, replacement of micro electrode assembly 14.

In accordance with the present invention sensors preferably comply withalarm specifications set forth in UL 2034.

It will be understood that, unless otherwise specified, the chemicalreagents and compounds provided herein below, or their precursors, areavailable from common commercial chemical vendors, such as Sigma-AldrichChemical Co., of Milwaukee, Wis.

The invention is further described by the following examples.

Example 1 Preparation of Tetranitro(Cobalt Phthalocyanine), CoPc(NO₂)₄

A 250 mL 3-neck round bottom flask fitted with a thermocouple and refluxcondenser was charged with cobalt sulfate heptahydrate (3.4 g, 12.1mmol), 4-nitrophthalic acid (9.25 g, 43.8 mmol), urea (15 g, 250 mmol),ammonium chloride (1.13 g, 21.1 mmol), and ammonium molybdatetetrahydrate (0.14 g, 0.1 mmol) in 6.5 mL nitrobenzene. The mixture washeated to 185° C. for 5 hours under a blanket of nitrogen. The reddishmixture quickly turned dark blue. The solid was collected on a glassfrit filter and washed with 350 mL of warm methanol to remove residualnitrobenzene. The solid was then boiled in 150 mL 1M HCl saturated withNaCl for five minutes. The liquid was removed by filtration then thesolid was added to 150 mL 1M NaOH plus 50 g NaCl. The mixture was heatedto 90° C. under a light argon flow until ammonia was no longer evolvedas checked with damp pH paper. The mixture was cooled and the solidcollected by filtration. The solid was alternately washed three timeswith 1.0M HCl (60 mL) then 1.0M NaOH (60 mL). This was followed bywashing with 400 mL water. The solid was dried in a vacuum oven at 70°C. for 24 hours. Yield: 7.86 g=86%

Example 2 Preparation of Tetramino Cobalt Phthalocyanine, CoPc(NH₂)₄

A 50 mL round bottom flask fitted with a condenser was charged withCoPc(NO₂)₄ (0.68 g, 0.9 mmol) and sodium sulfide monohydrate (10.4 g,43.4 mmol) in 15 mL water. The mixture was stirred at room temperaturefor 72 hours then heated to reflux for 24 hours. After cooling to roomtemperature, a blue solid was collected by filtration. The solid waswashed with 50 mL water then it was dissolved in 100 mL 1M HCl. A smallamount of insoluble material was removed by filtration. The filtrate wascollected and the pH was raised to >11 by the very careful addition ofNaOH pellets. A large amount of greenish-blue precipitate formed whichwas collected by filtration and washed with copious amounts of wateruntil the filtrate was neutral by pH paper. The solid was dried in avacuum oven at 70° C. for 24 hours. Yield: 0.28 g=49%

Example 3 Cobalt Phthalocyanine Deposited and Thermally Treated onMulti-walled Carbon Nanotubes, CoPc-MWNT

A 50 mL round bottom flask was charged with 100 mg cobalt phthalocyanine(Aldrich), 200 mg MWNT(Cheap Tubes Inc., 10-20 nm diameter, 10-30 μMlength, >95% purity), and 25 mL dry DMF. The mixture was sonicated for40 minutes (750 W, 38% amplitude, 20 seconds on/10 seconds off pulse).The DMF was removed by rotary evaporation. The purple tinted tubes weredried in a vacuum oven at 70° C. for 24 hours. The CoPc deposited tubes(299 mg) were placed in a fused quartz boat and inserted into a tubefurnace. The furnace was purged with argon (275 mL/min) for 45 minutes.The argon flow was reduced to 100 mL/min then the furnace was heated to700° C. That temperature was held for two hours then cooled to roomtemperature. Final yield: 273 mg. Thermogravimetric analysis, TGAyielded 6.03 wt % residue.

Example 4 Metal-Free Phthalocyanine Deposited and Thermally Treated onMulti-wall Carbon Nanotubes, Pc-MWNT

A 250 mL conical flask was charged with 222 mg 29H,31H-phthalocyanine(Aldrich), 667 mg MWNT (Cheap Tubes Inc., 10-20 nm diameter, 10-20 μMlength, >95% purity), and 150 mL anhydrous DMF. The mixture wassonicated for 30 minutes (750 W, 50% amplitude, 30 seconds on/10 secondsoff pulse) while in an ice bath. The suspension was poured into 800 mL1:1 hexane/ethyl ether with vigorous stirring. The solid was collectedon a 0.45 μM PTFE membrane filter and washed with two portions of 250 mL1:1 hexane/ethyl ether. The material was transferred to a fused quartzboat and placed in a tube furnace. The solid was dried at 100° C. undera flow of anhydrous argon at 235 mL/min for 90 minutes. The argon flowwas reduced to 65 mL/min then the temperature ramped to 700° C. and heldthere for 120 minutes. After cooling to room temperature, 730 mg ofcatalyst was yielded.

Example 5 Sample Preparation, CoPc(NH₂)₄-MWNT/Nafion

5.0 mg CoPc(NH₂)₄-MWNT was suspended in 10.0 mL 1:1 ethanol/water. Themixture was sonicated for 10 minutes (750 W, 30% amplitude, 20 sec on/10sec off pulse). 40.0 μL 5% Nafion solution was added. The suspension wassonicated for an additional 2 minutes under the above conditions. 20.0μL of the black suspension was carefully deposited onto only the glassycarbon of a RDE electrode (Pine Instruments, 5.0 mm diameter) andallowed to dry. All samples were prepared in a similar manner.

Experimental Parameters

0.5M H₂SO₄ was used as an electrolyte. The counter electrode wasplatinum mesh and the reference electrode was a double junction Ag/AgClelectrode.

Initial Activities for Oxygen Reduction by Cyclic Voltammetry

The electrolyte was purged with oxygen then the sample was scanned from1.0 to −0.2V at 10 mV/s to determine the initial activity of the sample.This was repeated for several cobalt phthalocyanines on both MWNT andcommercially available XC72 carbon. A state of the art fuel cellcatalyst, 10 wt % Pt/XC72 (E Tek) was also included as was 5 wt %Pt/carbon (Aldrich). See FIG. 6 for results. Activity is determined bythe onset of current. As shown in FIG. 6, the 10 wt % Pt/XC72 was themost active though all the cobalt based catalysts were nearly as active,which verifies that a broad spectrum of novel cobalt based catalystsexhibit sufficient electrochemical activity. The nitrogen dopednanotubes, N-MWNT, showed very little activity.

Carbon Monoxide Activity

Unlike platinum based catalysts which are active in both the reductionof oxygen and the oxidation of various fuels, such as hydrogen,methanol, or carbon monoxide, the cobalt-nitrogen based catalystsdisclosed herein have been developed to operate only as ORR catalysts.To check for activity for the catalytic oxidation of carbon monoxide, asample of CoPc(NO₂)₄-XC72/Nafion was studied. Identical conditions tostudy the ORR activity were used with the exception that the electrolytewas purged with carbon monoxide for 30 minutes. The sample was scannedfrom −0.2 to 1.0 to −0.2V at mV/s. See FIG. 7 for results. The scanunder carbon monoxide was almost identical to that under argon, an inertgas. There was essentially no activity. Subsequent scanning under carbondioxide also revealed inactivity for the reduction of carbon dioxide.

Example 6 Molecular Activation of MWNT

10 wt % PCA-MWNT: A 500 mL round bottom flask was charged with 130 mg1-pyrenecarboxylic acid (PCA) (Sigma-Aldrich), 1.18 g MWNT (Cheap TubesInc, 10-20 nm diameter, 10-30 μM length, >95% purity), and 180 mLethanol. The mixture was sonicated for one hour (750 W, 50% amplitude,20 s on/10 s off pulse). The suspension was allowed to sit for 48 hoursthen the solid was collected by filtration on a 0.2 μM PTFE membranefilter. The solid was washed with 120 mL ethanol then dried under vacuumat 70° C. for 24 hours. TGA yielded 0.7% ash, a typical value for thisbatch of MWNT.

Example 7 Preparation of Anodic Catalyst I

10.5 wt % Pt/PCA-MWNT: A 50 mL Erlenmeyer flask was charged with 50 mg10 wt % PCA-MWNT in 25 mL 5% H₂O in ethylene glycol. The suspension wassonicated for 10 minutes (750 W, 30% amplitude, 20 s on/10 s off pulse).To this suspension was added 14 mg H₂PtCl₆ (Sigma-Aldrich) in 5 mL 5%H₂O in ethylene glycol over 3 minutes with vigorous stirring. After 10minutes of stirring, the suspension was sonicated under the aboveconditions for 3 minutes. A few Teflon chips were added to thesuspension which was then heated in a microwave for two minutes (2450MHz, 1000 W, 50% power) under nitrogen. After cooling to roomtemperature, the suspension was diluted with 30 mL ethanol and the solidwas collected by filtration on a 0.2 μM PTFE membrane filter. Theproduct was washed with 100 mL ethanol then dried in a vacuum oven at70° C. for 16 hours. TGA yielded 10.5 wt % Pt.

Example 8 Preparation of Anodic Catalyst II

9.6 wt % Pt/PCA-MWNT: A 1-L Erlenmeyer flask was charged with 1.25 gPCA-MWNT in 540 mL 5% H₂O in ethylene glycol. The modified nanotubeswere suspended in the solvent by sonication for 120 minutes (750 W, 50%amplitude, 30 s on/10 s off pulse). The suspension was transferred to a1-L round bottom flask. A magnetic stir bar was added to the flask.Under vigorous stirring, 347 mg H₂PtCl₆ (Sigma-Aldrich, 40 wt % Pt) in10 mL 5% H₂O in ethylene glycol was added over one minute. Stirring wascontinued for 2 hours. The flask was then fitted with a reflux condenserunder a blanket of nitrogen and placed in microwave (Milestone Ethos,1200 W). Under full power and magnetic stirring, the suspensiontemperature was ramped to reflux at 165° C. in 4 minutes and was heldthere for an additional 8 minutes. After cooling to room temperature,the suspension was diluted with 500 mL ethanol then collected byfiltration on a 0.45 μM PTFE membrane filter. The filtrate was washedwith two portions of 200 mL ethanol each followed by drying under vacuumat 70° C. for 16 hours. TGA yielded 9.6 wt % Pt. Powder XRD on asimilarly prepared sample yielded an average Pt nanoparticle size of 3.3nm by examination of the Pt[200] peak.

Example 9 Preparation of Anode Ink

10.2 wt % Pt/PCA-MWNT was finely ground using an agate mortar andpestle. A 125 mL Erlenmeyer flask was charged with 254 mg 10.2 wt %Pt/PCA-MWNT, 6.5 g glycerol (Sigma-Aldrich), 865 mg water (18.2 MΩ-cm,Millipore), and 8.5 g 1-propanol (Sigma-Aldrich). Approximately 60 g ofexcess 1-propanol was then added. The catalyst was suspended bysonication for 60 minutes (750 W, 50% amplitude, 30 s on/10 s offpulse). 850 mg Nafion solution (10 wt % in H₂O, Fuel Cell Store) dilutedin 11 g 1-propanol was added to the suspension. The suspension wassonicated for an additional five minutes under the conditions listedpreviously. A magnetic stir bar was placed in the flask and thesuspension was vigorously stirred for 16 hours. The suspension wastransferred to a tared bottle and concentrated using heat and a flow ofdry argon, until 2 wt % solids was reached. 80 μL tetrabutylammoniumhydroxide (TBA-OH) (1.0M in methanol, Sigma-Aldrich) was added to covertthe acidic Nafion to the heat stable TBA⁺ form. After removal ofvolatiles, an electrode consisting of 67 wt % (Pt/PCA-MWNT) and 33 wt %Nafion results.

Example 10 Preparation of Cathode Ink

A 250 mL pear shaped flask was charged with 150 mg CoPc(NO₂)₄-MWNT, 150mg MWNT (Cheap Tubes Inc., Brattleboro, Vt.), 83 mg PTFE dispersion(0.1-1.0 μM, 60 wt % in H₂O, Ion-Power Inc.), 9.60 g glycerol, 2.09 gH₂O, and 12.18 g 1-propanol. Approximately 125 g of excess 1-propanolwas added. The mixture was sonicated for 120 minutes (750 W, 50%amplitude, 30 s on/10 s off pulse) while cooled in an ice bath. Amagnetic stirbar was added to the suspension and under vigorousstirring, 668 mg Nafion solution (15 wt % in aqueous alcohols, LiquionLQ-1115, Ion-power Inc., New Castle, Del.) was added. The suspension wasmixed for two hours then sonicated for five minutes under the aboveconditions. 141 μL tetrabutylammonium hydroxide solution (1.0M inmethanol, Sigma-Aldrich) was added then the suspension was sonicated forfive minutes under the above conditions. The suspension was concentratedby rotary evaporation then transferred to a tared bottle andconcentrated further with heat until a 2 wt. % solids suspension wasachieved. After removal of volatiles, an electrode consisting of 30.0 wt% (33% Co(Pc(NO₂)₄/MWNT), 30.0 wt % MWNT, 20 wt % Nafion, and 20 wt %PTFE results.

Example 11 Preparation of Na⁺-Nafion 117

A 8 cm×11 cm piece of Nafion 117 membrane (Dupont) was placed in 275 mLwater purified by reverse osmosis at 90° C. for 120 minutes. Themembrane was quickly rinsed with water then placed in 275 mL 3% H₂O₂ at90° C. for 60 minutes. The membrane was rinsed with water for 10 minutesthen converted to the Na⁺ form by placing it into 275 mL 1M NaOH for 120minutes. The membrane was rinsed with water for 15 minutes and allowedto air dry. After cutting into pieces of the desired size, it was storedat room temperature in water (18.2 MΩ-cm, Millipore).

Example 12 Decal Preparation

A stamp of pattern similar to FIG. 3C is machined from a 1 cm thickblock of Teflon. The stamp dimensions are approximately 1 cm×0.7 cm witha 0.1 cm gap between the two electrodes. A light coating of PTFE moldrelease is applied to the stamp. Using a micropipette, anodic ink (10%Pt/PCA-MWNT/Nafion, 2 wt % solids) is applied to the anode in a volumeof 100-1000 μL/cm², preferably 150-300 μL/cm². Using a micropipette,cathodic ink (1:1(33% Co(Pc(NO₂)₄/MWNT)/MWNT/Nafion, 2 wt % solids) isapplied to the cathode in a volume of 100-1000 μL/cm², preferably150-300 μL/cm². The stamp is dried at 50-250° C., preferably 180° C. Aflow of dry nitrogen across the part or application of vacuum may beused in addition to heat to speed drying of the ink. A second layer ofanodic ink is applied to the anode for a total of two. Three additionallayers of cathodic ink is applied to the cathode for a total of four.Identical drying procedures as listed previously are used to dry eachindividual layer of ink.

Example 13 Heat Pressing Decal onto Ion-Exchange Membrane

A 1.5 cm×1.5 cm piece of Nafion 117 membrane (Dupont) in the Na⁺ form isplaced on top of the decal coated Teflon stamp. A piece of PTFE coatedstainless steel sheet is placed on top the ion-exchange membrane. Theassembly is then placed in a heat press set to 195° C. Pressure (47 atm)is applied to the assembly for 120 seconds. The assembly is removed fromthe press and allowed to cool. The ion-exchange membrane is slowlypeeled away from the Teflon stamp. Ideally all of the ink will havetransferred from the stamp to the membrane.

Example 14 MEA Acidification and Rehydration

The prepared MEA is placed into a volume of 20-100 mL of 0.1M H₂SO₄. Itis heated to 60-100° C., preferably 80-90° C. for 1-16 hours. The 0.1MH₂SO₄ is decanted and this step may optionally be repeated. The MEA isthen rinsed with H₂O purified by reverse osmosis and placed into avolume of 20-100 mL of H₂O. It is heated to 60-100° C., preferably80-90° C. for 1-16 hours. The MEA is rinsed a final time with H₂Opurified by reverse osmosis then stored flat between two absorbentsheets until use.

Example 15 Addition of MWNT to Cathodic Inks

A stamped electrode on a Nafion membrane prepared from a 2 wt % solidsink containing 1.5 wt % (20% Co(Pc(NO₂)₄/MWNT) and 0.5 wt % Nafionresulted in an average resistance of 35-70 kΩ/cm. To reduce theresistance of the electrodes, some of the catalytic carbon material wasreplaced with pure MWNT. A stamped electrode of similar cross-sectionalarea on a Nafion membrane was prepared from a 2 wt % solids inkcontaining 0.75 wt % (33% Co(Pc(NO₂)₄/MWNT), 0.75 wt % MWNT (Cheap TubesInc., 10-20 nm diameter, 10-30 μM length, >95% purity) and 0.5 wt %Nafion. The average resistance was 2 kΩ/cm. Both electrodes containedapproximately the same amount of cobalt-N₄ catalyst. It was discoveredthat that the pyrolysis step dramatically decreased the conductivity ofthe MWNT carbon support—possibly due to coating the tubes withCoPc(NO₂)₄ which is then pyrolyzed to form a catalytically active butelectrically insulating layer of partially-graphitized carbon andnitrogen along with a small amount of cobalt. This coating which has alower electronic conductivity than the original, highly graphitic MWNT.By blending the pyrolyzed catalyst with pure MWNT, the electronicconductivity can be enhanced without affecting the catalytic activity.

Example 16 Addition of PTFE to Cathodic Inks

In the cathode of a fuel cell or sensor, H₂O is typically generated. Athigh currents normally found in fuel cells, this H₂O can cause floodingof the pores within the electrode and effectively reduce the amount ofoxygen that can reach the catalyst active sites. Known solutions includecareful regulation of the flow rate and/or humidity in the oxygen or airfeed which requires a sophisticated level of control and supplementaryequipment. Though sensors never operate under high current conditionslike fuel cells, they have near zero rates of airflow which limits theremoval of excess water by evaporation. Sensors are also open systems,exposed to the environment, and must deal with variations in ambienthumidity. As such, a sophisticated level of control and supplementaryequipment is not an option in a typical sensor application. A simplifiedmethod of addressing the removal of excess H₂O in gas sensors is to makethe electrode more hydrophobic by the addition of PTFE. Suitable PTFEaddition techniques are provided in, for example, U.S. Pat. No.6,800,391—which is hereby incorporated herein by reference in itsentirety—including all references cited therein. Moreover, with theaddition of PTFE, pore sizes within the electrode and the availabilityof the gas to the active site can be controllably modified.

Example 17 Performance of Sensors I and II

As is shown in FIGS. 8A and 8B, both Sensors I and II, which werefabricated in accordance with the present invention, exhibited welldefined performance characteristics toward compliance with alarmspecifications set forth in UL 2034.

Example 18 Sensor Calibration

As is shown in FIG. 9, sensors prepared in accordance with the presentinvention can surprisingly be calibrated with an ideal two-pointcalibration. It will be understood that normal sensor calibrationsrequire mathematical fits having computations substantially more complexthan the displayed linear calibration.

While the invention has been described in detail herein in accordancewith certain preferred embodiments thereof, many modifications andchanges therein may be effected by those skilled in the art.Accordingly, it is our intent to be limited only by the scope of theappending claims and not by way of details and instrumentalitiesdescribing the embodiments shown herein.

1-25. (canceled)
 26. An electrochemical carbon monoxide sensor for usein a gas/fire detector, said electrochemical carbon monoxide sensorbeing exposed to a sample gas during normal sensing operation,comprising: a housing, wherein the housing comprises a first sidewall, atop wall, and a bottom wall, and wherein the first sidewall, the topwall and the bottom wall define a containment region for containing amicro electrode assembly therein, and further wherein the top wall ofthe housing comprises a gaseous diffusion aperture; wherein the microelectrode assembly comprises an anode, a cathode and an ion conductingmembrane, and wherein the anode and the cathode are discrete elements,and wherein the ion conducting membrane permits ion transport betweenthe anode and the cathode, and wherein the ion conducting membraneprevents electron conduction between the anode and the cathode; whereinthe cathode comprises an oxygen reduction catalyst that comprises apyrolysis product of a carbonaceous material and an organometalliccoordination complex of a transition metal with a nitrogen containingmacrocycle that has been deposited on the carbonaceous material; whereinthe cathodic material does not materially exhibit catalytic activity forthe oxidation of carbon monoxide; and wherein the anode and the cathodeare configured to be exposed to the same sample gas during normalsensing operation.
 27. The electrochemical carbon monoxide sensoraccording to claim 26, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of the group comprisingsubstituted transition metal porphyrins, unsubstituted transition metalporphyrins, substituted transition metal tetrabenzoporphyrins,unsubstituted transition metal tetrabenzoporphyrins, substitutedtransition metal tetraphenylporphyrins, unsubstituted transition metaltetraphenylporphyrins, substituted transition metal tetraazaporphyrins,unsubstituted transition metal tetraazaporphyrins, substitutedtransition metal tetraazamacrocycles, unsubstituted transition metaltetraazamacrocycles, substituted transition metal phthalocyanines,unsubstituted transition metal phthalocyanines, substituted transitionmetal naphthalocyanines, unsubstituted transition metalnaphthalocyanines, substituted transition metal bis(phthalocyanines),unsubstituted transition metal bis(phthalocyanines), substitutedtransition metal bis(naphthalocyanines), unsubstituted transition metalbis(naphthalocyanines), or combinations thereof.
 28. The electrochemicalcarbon monoxide sensor according to claim 26, wherein the organometalliccoordination complex comprises the pyrolysis product of at least one ofthe group comprising substituted cobalt porphyrins, unsubstituted cobaltporphyrins, substituted cobalt tetrabenzoporphyrins, unsubstitutedcobalt tetrabenzoporphyrins, substituted cobalt tetraphenylporphyrins,unsubstituted cobalt tetraphenylporphyrins, substituted cobalttetraazaporphyrins, unsubstituted cobalt tetraazaporphyrins, substitutedcobalt tetraazamacrocycles, unsubstituted cobalt tetraazamacrocycles,substituted cobalt metal phthalocyanines, unsubstituted cobaltphthalocyanines, substituted cobalt naphthalocyanines, unsubstitutedcobalt naphthalocyanines, substituted cobalt bis(phthalocyanines),unsubstituted cobalt bis(phthalocyanines), substituted cobaltbis(naphthalocyanines), unsubstituted cobalt bis(naphthalocyanines), orcombinations thereof.
 29. The electrochemical carbon monoxide sensoraccording to claim 26, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of a compound, astructural isomer of a compound, and mixtures of isomers of compounds,represented by the following structure:

wherein M comprises a transition metal ligated by a tetraazamacrocycle.30. The electrochemical carbon monoxide sensor according to claim 26,wherein the organometallic coordination complex comprises the pyrolysisproduct of at least one of a compound, a structural isomer of acompound, and mixtures of isomers of compounds, represented by thefollowing formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₆ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof.
 31. The electrochemical carbon monoxide sensoraccording to claim 26, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of a compound, astructural isomer of a compound, and mixtures of isomers of compounds,represented by the at least one of the following formulae:


32. The electrochemical carbon monoxide sensor according to claim 26,wherein the organometallic coordination complex comprises the pyrolysisproduct of at least one of a compound, a structural isomer of acompound, and mixtures of isomers of compounds, represented by thefollowing formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₂ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof.
 33. The electrochemical carbon monoxide sensoraccording to claim 26, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of a compound, astructural isomer of a compound, and mixtures of isomers of compounds,represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₀ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof.
 34. The electrochemical carbon monoxide sensoraccording to claim 26, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of a compound, astructural isomer of a compound, and mixtures of isomers of compounds,represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₈ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof.
 35. The electrochemical carbon monoxide sensoraccording to claim 26, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of a compound, astructural isomer of a compound, and mixtures of isomers of compounds,represented by the following formula:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; wherein R₁-R₈are the same or different and comprise H, NO₂, NH₂, an amine, CO₂H, anester, an alkyl group containing approximately 1 to approximately 10carbon atom(s), an ether, SH, a thioether, or combinations thereof. 36.The electrochemical carbon monoxide sensor according to claim 26,wherein the organometallic coordination complex comprises the pyrolysisproduct of at least one of a compound, a structural isomer of acompound, and mixtures of isomers of compounds, selected from the groupconsisting of formulae II-VI and combinations thereof: wherein formulaII is;

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₆ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof; wherein formula III is;

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₁₂ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof; wherein formula IV is;

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₀ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof; wherein formula V is;

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; whereinR₁-R₂₈ are the same or different and comprise H, NO₂, NH₂, an amine,CO₂H, an ester, an alkyl group containing approximately 1 toapproximately 10 carbon atom(s), an ether, SH, a thioether, orcombinations thereof; and wherein formula VI is:

wherein M comprises Co, Cu, Fe, Ir, Ni, Pd, Rh, Ru, or Zn; wherein R₁-R₈are the same or different and comprise H, NO₂, NH₂, an amine, CO₂H, anester, an alkyl group containing approximately 1 to approximately 10carbon atom(s), an ether, SH, a thioether, or combinations thereof. 37.The electrochemical carbon monoxide sensor according to claim 26,wherein the carbonaceous material is selected from the group comprisinggraphene, graphite, amorphous carbon, carbon nanotubes, carbon fibers,or combinations thereof.
 38. The electrochemical carbon monoxide sensoraccording to claim 26, further comprising a desiccant for retaining aliquid.
 39. An electrochemical carbon monoxide sensor for use in agas/fire detector, said electrochemical carbon monoxide sensor beingexposed to a sample gas during normal sensing operation, comprising: ahousing, wherein the housing comprises a first sidewall, a top wall, anda bottom wall, and wherein the first sidewall, the top wall and thebottom wall define a containment region for containing a micro electrodeassembly therein, and further wherein the top wall of the housingcomprises a gaseous diffusion aperture; wherein the micro electrodeassembly comprises an anode, a cathode and an ion conducting membrane,and wherein the anode and the cathode are discrete elements, and whereinthe ion conducting membrane permits ion transport between the anode andthe cathode, and wherein the ion conducting membrane prevents electronconduction between the anode and the cathode; wherein the cathodecomprises an oxygen reduction catalyst that comprises a pyrolysisproduct of a carbonaceous material and an organometallic coordinationcomplex of a transition metal with a nitrogen containing macrocycle thathas been deposited on the carbonaceous material; and wherein the anodeand the cathode are configured to be exposed to the same sample gasduring normal sensing operation.
 40. The electrochemical carbon monoxidesensor according to claim 39, wherein the organometallic coordinationcomplex comprises the pyrolysis product of at least one of the groupcomprising substituted transition metal porphyrins, unsubstitutedtransition metal porphyrins, substituted transition metaltetrabenzoporphyrins, unsubstituted transition metaltetrabenzoporphyrins, substituted transition metaltetraphenylporphyrins, unsubstituted transition metaltetraphenylporphyrins, substituted transition metal tetraazaporphyrins,unsubstituted transition metal tetraazaporphyrins, substitutedtransition metal tetraazamacrocycles, unsubstituted transition metaltetraazamacrocycles, substituted transition metal phthalocyanines,unsubstituted transition metal phthalocyanines, substituted transitionmetal naphthalocyanines, unsubstituted transition metalnaphthalocyanines, substituted transition metal bis(phthalocyanines),unsubstituted transition metal bis(phthalocyanines), substitutedtransition metal bis(naphthalocyanines), unsubstituted transition metalbis(naphthalocyanines), or combinations thereof.
 41. The electrochemicalcarbon monoxide sensor according to claim 39, wherein the organometalliccoordination complex comprises the pyrolysis product of at least one ofthe group comprising substituted cobalt porphyrins, unsubstituted cobaltporphyrins, substituted cobalt tetrabenzoporphyrins, unsubstitutedcobalt tetrabenzoporphyrins, substituted cobalt tetraphenylporphyrins,unsubstituted cobalt tetraphenylporphyrins, substituted cobalttetraazaporphyrins, unsubstituted cobalt tetraazaporphyrins, substitutedcobalt tetraazamacrocycles, unsubstituted cobalt tetraazamacrocycles,substituted cobalt metal phthalocyanines, unsubstituted cobaltphthalocyanines, substituted cobalt naphthalocyanines, unsubstitutedcobalt naphthalocyanines, substituted cobalt bis(phthalocyanines),unsubstituted cobalt bis(phthalocyanines), substituted cobaltbis(naphthalocyanines), unsubstituted cobalt bis(naphthalocyanines), orcombinations thereof.
 42. The electrochemical carbon monoxide sensoraccording to claim 39, wherein the organometallic coordination complexcomprises the pyrolysis product of at least one of a compound, astructural isomer of a compound, and mixtures of isomers of compounds,represented by the following structure:

wherein M comprises a transition metal ligated by a tetraazamacrocycle.43. An electrochemical carbon monoxide sensor for use in a gas/firedetector, said electrochemical carbon monoxide sensor being exposed to asample gas during normal sensing operation, comprising: a housing havinga containment region for containing a micro electrode assembly therein,and further wherein the housing comprises a gaseous diffusion aperture;wherein the micro electrode assembly comprises an anode, a cathode andan ion conducting membrane, and wherein the ion conducting membranepermits ion transport between the anode and the cathode, and wherein theion conducting membrane prevents electron conduction between the anodeand the cathode; wherein the cathode comprises an oxygen reductioncatalyst that comprises a pyrolysis product of a carbonaceous materialand an organometallic coordination complex of a transition metal with anitrogen containing macrocycle that has been deposited on thecarbonaceous material; and wherein the anode and the cathode areconfigured to be exposed to the same sample gas during normal sensingoperation.
 44. The electrochemical carbon monoxide sensor according toclaim 43, wherein the organometallic coordination complex comprises thepyrolysis product of at least one of the group comprising substitutedtransition metal porphyrins, unsubstituted transition metal porphyrins,substituted transition metal tetrabenzoporphyrins, unsubstitutedtransition metal tetrabenzoporphyrins, substituted transition metaltetraphenylporphyrins, unsubstituted transition metaltetraphenylporphyrins, substituted transition metal tetraazaporphyrins,unsubstituted transition metal tetraazaporphyrins, substitutedtransition metal tetraazamacrocycles, unsubstituted transition metaltetraazamacrocycles, substituted transition metal phthalocyanines,unsubstituted transition metal phthalocyanines, substituted transitionmetal naphthalocyanines, unsubstituted transition metalnaphthalocyanines, substituted transition metal bis(phthalocyanines),unsubstituted transition metal bis(phthalocyanines), substitutedtransition metal bis(naphthalocyanines), unsubstituted transition metalbis(naphthalocyanines), or combinations thereof.
 45. The electrochemicalcarbon monoxide sensor according to claim 43, wherein the organometalliccoordination complex comprises the pyrolysis product of at least one ofa compound, a structural isomer of a compound, and mixtures of isomersof compounds, represented by the following structure:

wherein M comprises a transition metal ligated by a tetraazamacrocycle.